This invention relates to novel cyclic phosphonate esters which are useful for flame-proofing thermoplastic polymers, particularly polyolefins and polyesters.
The use of cyclic esters of diphosphonic acids for flame-proofing thermoplastic polymers such as polyurethanes, polyesters, polyacrylates, polymethacrylates, polyamides and polyvinyl chloride is disclosed by Baranauckas et al. in U.S. Pat. Nos. 3,511,857 and 3,515,776. Baranauckas et al. also disclose the use of acrylic esters of diphosphonic acids in U.S. Pat. Nos. 3,538,196 and 3,737,397 for similar purposes. However, the diphosphonate esters of Baranauckas et al. must contain from 2 to about 32 hydroxyl groups for further reaction with other polyfunctual intermediates in the polymer substrate, such as polybasic acids and anhydrides, alkyd resins, toluene diisocyanate, and the like.
The use of the cyclic ethylene and trimethylene esters of benzylphosphonic acid as flameproofing agents is disclosed in German Pat. No. 2,153,149.
U.S. Pat. No. 3,922,323 of Reese et al. discloses various halogen-containing cyclic phosphonate esters and their use as flame retardants for unsaturated polyesters. However, when these compounds are used as flame retardants for linear thermoplastic polyesters, they must be processed at much higher temperatures which results in the degradation of the polyester as evidenced by a drastic reduction in the melt viscosity of the polymer.
U.S. Pat. No. 3,890,409 of Mayerhoefer et al. discloses various cyclic diphosphates and diphosphites as flame retardants.
However, none of these materials has proven to be completely satisfactory and the search still continues for a satisfactory flame retardant compound for the flame-proofing of thermoplastic polymers, particularly polyolefins and polyesters.